g. Optic nerves. There is a superficial chiasma to these nerves in Elasmobranchii, Ganoidei, and Dipnoi.

h. Third nerve or oculomotor. It springs from the base of the mid-brain, but its superficial origin is hidden by the saccus vasculosus.

i. Fourth nerve or trochlearis. It arises, as in all Vertebrata, from the roof of the aquaeductus Sylvii behind the mid-brain, and in front of the cerebellum.

j. Roots of the fifth (trigeminus), seventh (facial or 'portio dura'), and eighth (auditory or 'portio mollis') nerves. The facial rises dorsally, the auditory posteriorly and ventrally, while the remaining roots belong to the trigeminus. This close connection of the nerves at their roots is generally found in Pisces. The trigeminus and facial are also closely connected in the Gasserian ganglion in Anura (Amphibia). The facial and auditory nerves also are often closely connected in Amphibia and Reptilia at their roots.

k. The roots of the vagus nerve or pneumogastric. The glossopharyngeal nerve forms the most anterior of these strands which rise close together from the medulla oblongata.

FIG. 4. Longitudinal section through an Elasmobranch embryo at a time when the neural canal, notochord, and alimentary canal are established. From Balfour, Development of Elasmobranch Fishes. London, 1878, Fig. 1 c, p. 58.

ep. Epiblast or superficial layer of cells formed from the blastoderm, and extending, with the mesoblast and hypoblast, round the ovum, to form the yolk sac. This growth or extension is greatest at the anterior end of the embryo. It is slight at the posterior end, where was situated the aperture of invagination, blastopore, or anus of Rusconi, beneath the embryonic rim.

nc. Neural canal which is formed first as a groove in the blastoderm (medullary groove), and is then converted into a canal by the closure of the sides of this groove. The medullary groove extends to the posterior end of the blastoderm and the dorsal edge of the blastopore. On its closure to form the neural canal a pore persists at this spot; and when the dorsal edge of this pore, continuous with the edge of the blastopore, grows down over the yolk, a communication is left between the neural and alimentary canals.

x. Communication between the two canals named, known as the neuren-teric canal. It is a common feature in vertebrate development, and lies, as a rule, behind the spot where the anus is formed. Hence there is a post-anal extension, not only of the neural, but also of the alimentary canal. See Balfour, Comp. Embryology, vol. ii. p. 267, 268; p. 634-636. Cf. Spencer, Q. J. M. Suppl. 1885, p. 126-7.

ch. Notochord. A rod of cells differentiated at an early period from the layer of cells below the medullary groove, and therefore probably to be considered as of hypoblastic origin.

m. Mesoblast.

al. Alimentary canal, roofed in by cells which are derived together with the mesoblast from the original mass of cells underlying the epiblast. The mesoblast cells are wanting immediately below the medullary groove, but form two plates to either side of it. Both mesoblast and hypoblast extend with the epiblast round the non-segmented part of the ovum or yolk to form the yolk sac.

n. Nuclei in the yolk. These nuclei appear spontaneously (?) in the protoplasm of the yolk, i. e. in the network between the secondary yolk or deutoplasm. A portion of protoplasm separates round each nucleus. This process begins at an early stage, and the cells thus formed are added at first to the lower layer of blastoderm cells, and subsequently to the hypoblast. They thus contribute to its growth, and especially to the formation of the floor of the alimentary canal.

The chief works on the Anatomy of the Skate and Elasmobranchii are the following: Anatomy of Skate. T. J. Parker, Zootomy, London, 1884.

Zoological account of Rays. Cf. Day, Fishes of Great Britain and Ireland, ii. 1884, p. 336. Couch, Fishes of British Islands (coloured figures), i. 1862, p. 78. Bell, British Fishes, ii. 1841, p. 550. Gunther, British Museum Catalogue of Fishes, viii. 1870, p. 434.

Skull. W. K. Parker, Tr. Z. S. x. 1879. Visceral arches. Dohrn, Mitth. Zool. Stat. Naples, vi. 1885.

Paired fins. Balfour, P. Z. S. 1881. Paired and azygos fins. Mivart, Tr. Z. S. x. 1879 (see general remarks). Azygos fins. Mayer, Mitth. Zool. Stat. Naples, vi. 1885.

Exoskeleton. Hertwig, J. Z. viii. 1874. Benda, A. M. A. xx. 1882.

Teeth. Tomes, Ph. Tr. 1876. Id. Dental Anatomy, ed. 2, 1882, p. 215.

Central nervous system. Rohon, Dk. Akad. Wien, xxxviii. 1878. Viault, A. Z. Expt. v. 1876. Systeme ganglionaire. Vignal, A. Z. Expt. (2), i. 1883, p. xvii. Spinal cord, Stieda, Z. W. Z. xxiii. 1873; Sanders, P. R. S. xl. 1886.

Cranial nerves. Marshall, Q. J. M. xxi. 1881. Cf. Beard, Branchial sense-organs, Q. J. M. xxv. 1885. Vagus nerve. Rohon, Arb. Zool. Inst. Wien, i. 1878.

Gustatory organs. Todaro, A. Z. Expt. ii. 1873. Organs of lateral line. Solger, A. M. A. xvii. 1880. Do. and ampullae. Merkel, Endigungen der sensibeln Nerven in der Haut, Rostock, 1880.

Intestinal spiral valve. T. J. Parker, Tr. Z. S. xi. 1880. Rectal gland. Blanchard, Journal de l'Anat. et Physiol. xiv. 1878. Function of do. Id. Bull. Soc. Zool. de France, vii.

Rudimentary gill slits. Van Bemmelen, Mitth. Zool. Stat. Naples, vi. Histology of gills. Droscher, A. N. 48, 1882. Rudiment of air bladder. Miklucho-Maclay, J. Z. in. 1867.

Valves in conus arteriosus of Elasmobranchii and Ganoidei. Stohr, M. J. ii. 1876; for Raja Batis, see p. 219.

Venous system of Skate. T. J. Parker, Trans. New Zealand Institute, xiii. 1880.

Urinary apparatus and genital ducts. Balfour, Development of Elasmobranch Fishes, 1878, pp. 249-286.

Ovary. Balfour, Q. J. M. xviii. 1878. Cf. J. Muller, Berlin, Abhandl. 1840, for forms of ovum, and Gunther, Study of Fishes, 1880.

Copulatory organs. Petri, Z. W. Z. xxx. 1878; see Bolau, Z. W. Z. xxxv. 1881; and Schneider, Zool. Beitrage, i. 1. 1883.

Port abdominales. Bridge, Journal of Anat. and Physiol. 1879. Cf. Ayers and Gegenbaur, M. J. x. 1884.

Chemistry of horny fibres in Mustelus and egg-case of Scyllium. Krukenberg, Mitth. Zool. Stat. Naples, vi. 1885.